1. Field of the Invention
The invention relates to a ball screw apparatus.
2. Description of Related Art
There has been proposed a ball screw apparatus with a ball train interposed between a ball track in an outer periphery of a ball screw shaft and a ball track in an inner periphery of a ball nut. In the ball screw apparatus, a coil spring is interposed between a ball at an end of the ball train and a pocket-like recessed portion formed on the ball nut (see, for example, U.S. Patent Application Publication No. 2012-0018262 A1 (US 2012-0018262 A1)). In such a non-circulating ball screw apparatus, the coil spring does not contract when the ball screw shaft is rotationally driven with a low axial load imposed on the ball screw shaft. Thus, the balls in the ball train do not move relative to the ball nut. Therefore, the ball screw moves with the balls in the ball train sliding on the ball screw shaft.
On the other hand, when the ball screw is rotationally driven with a high axial load imposed on the ball screw shaft, the coil spring contracts to allow the ball screw to move with the balls rolling with respect to both the ball nut and the ball screw shaft. Thus, efficient screw power transmission is achieved.
The coil spring is housed in a raceway between the ball screw shaft and the ball nut. Thus, when the ball screw shaft is rotationally driven, the shape of the coil spring is likely to be distorted. In particular, an end of the coil spring engaged directly with a recessed portion that is a stopper is likely to, for example, come into partial contact with the recessed portion and the position of the end of the coil spring is likely to be unstable. Consequently, the end of the coil spring may move toward the ball screw shaft and interfere with the ball screw shaft.
Thus, the inventors of the present application propose that the end of the coil spring be supported by the recessed portion via a stopper ball so that the orientation of the end of the coil spring is stabilized. Unfortunately, at the time of power transmission, the stopper ball may be caught, like a wedge, between the ball track of the ball screw shaft and the inner surface of the recessed portion of the ball nut, which are inclined in the direction of a lead angle. This may hinder the power transmission.